Typically in the case of using a vacuum thermal insulation material, a core material composed of glass wool is inserted into an envelope comprising a plurality of layers. After drawing a vacuum inside, the envelope is sealed by heating and melting the insertion opening, so as to prevent the vacuum from being broken by external factors and maintain the vacuum over a long period of time. The envelope is normally composed of a laminate obtained by layering a hot-melt layer, barrier layer and protective layer in that order starting from the inside. For example, Patent Document 1 discloses an envelope composed of a laminate comprising an outside hot-melt layer, a barrier layer, internal protective layer and external protective layer in this order. In addition, Patent Document 2 discloses a vacuum thermal insulation material further comprising a film on the outside, while Patent Document 3 discloses a vacuum thermal insulation material further comprising a second hot-melt portion which is formed on the outside of a hot-melt sealed portion.
The use of such vacuum thermal insulation materials as insulating materials that enhance energy-saving effects for the purpose of preventing global warming caused by carbon dioxide gas emissions has become increasingly popular in home appliances, and particularly refrigerators, and in recent years, these materials have been commercialized for use in applications other than refrigerators, such as bathtubs and bathtub covers in the field of housing equipment as well as the outer insulation of residential house walls.
In these applications, the vacuum thermal insulation materials are used at normal temperature or in a temperature range slightly higher than normal temperature. In the case of refrigerators, for example, vacuum thermal insulation materials are exposed to the temperature inside the refrigerator and the temperature outside the refrigerator. In the case of bathtubs, since vacuum thermal insulation materials are used on the insides of plastic bathtubs, the temperature to which vacuum thermal insulation materials are exposed is predicted to be lower than the hot water temperature, which is generally 42° C. or lower. In the case of residential house walls as well vacuum thermal insulation materials are exposed to temperatures generally up to about 40° C. even during the summer months, although varying according to orientation and location.
Although the use of such vacuum thermal insulation materials has increased in comparatively low temperature ranges, their use has not increased at high temperatures (ranging from higher than 60° C. to about 100° C.). In the case of automobiles, for example, since the temperature of an automobile roof is said to reach nearly 100° C. in the summer, residential house roofs are said to reach a temperature of about 60° C. in the summer and the temperature of hot water in hot water tanks is about 90° C., the use of vacuum thermal insulation materials in such applications is limited.
The reason why vacuum thermal insulation materials are not widely used at high temperatures is due to the considerable deterioration of insulating performance of the vacuum thermal insulation material at temperatures exceeding 40° C. Although an invention has been disclosed in which another material is inserted between a hot water tank and a vacuum thermal insulation material in order to reduce deterioration of the performance of the vacuum thermal insulation material caused by heat, deterioration of insulating performance of the vacuum thermal insulation material at high temperatures is unable to be prevented in this invention. Although the use of an extremely high-performance adsorbent is effective in inhibiting deterioration of insulating performance even in conventional vacuum thermal insulation materials, these adsorbents are extremely expensive and difficult to acquire. Although patent applications have been filed for use in automobiles or hot water tanks and the like in the manner of Patent Documents 4 and 5, vacuum thermal insulation materials capable of maintaining a vacuum at high temperatures still have room for improvement.